Physicochemical Strategies for Inhibition of Amyloid Fibril Formation: An Overview of Recent Advances

Inhibitors of amyloid fibril formation

Many diseases are caused by misfolded and denatured proteins, leading to neurodegenerative diseases. In recent decades researchers have developed a variety of compounds, including polymeric inhibitors and natural compounds, antibodies, and chaperones, to inhibit protein aggregation, decrease the toxic effects of amyloid fibrils, and facilitate refolding proteins. The causes and mechanisms of amyloid formation are still unclear, and there are no effective treatments for Amyloid diseases. This section describes research and achievements in the field of inhibiting amyloid accumulation and also discusses the importance of various strategies in facilitating the removal of aggregates species (refolding) in the treatment of neurological diseases such as chemical methods like as, small molecules, metal chelators, polymeric inhibitors, and nanomaterials, as well as the use of biomolecules (peptide and, protein, nucleic acid, and saccharide) as amyloid inhibitors, are also highlighted.

Electrospinning of bovine serum albumin-based nano-fibers: From synthesis to medical prospects; Challenges and future directions

Bovine serum albumin (BSA) is a globular non-glycoprotein that has gotten a lot of attention because of its unique properties like biocompatibility, biodegradability, non-immunogenicity, non-toxicity, and strong resemblance to the natural extracellular matrix (ECM). Given its robust mechanical properties, such as interfacial tension, conductivity, swelling resistance, and viscoelasticity, it can be concluded that it is an appropriate matrix for producing novel BSA-based nanoconstructs. Thus, simple analytic methods are required for accurately detecting BSA as a model protein in medical sciences and healthcare. Furthermore, the characteristics mentioned above aid BSA in the electrospinning process, which results in fibers conjugated with other polymers. Electrospun synthesis has recently received much attention for its ability to produce stable, biomimicking, highly porous, 3D BSA-derived nano-fibers. As a result, BSA-based nano-fibers have achieved exclusive developments in the medical sector, such as tissue engineering for the remodeling of damaged tissue or organ repair by creating artificial ones. Meanwhile, they could be used as drug delivery systems (DDS) for target-specific drug delivery, wound dressings, and so on. This study illustrates the structural and physicochemical properties of BSA and the determination of BSA using various methods, by citing recent reports and current developments in the medical field. Furthermore, current challenges and future directions are also highlighted.

Mushroom Polysaccharides as Potential Candidates for Alleviating Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a widespread and serious global public health burden, particularly among the older population. At present, effective therapies do not exist, despite the increasing understanding of the different mechanisms of NDs. In recent years, some drugs, such as galantamine, entacapone, riluzole, and edaravone, have been proposed for the treatment of different NDs; however, they mainly concentrate on symptom management and confer undesirable side effects and adverse reactions. Therefore, there is an urgent need to find novel drugs with fewer disadvantages and higher efficacy for the treatment of NDs. Mushroom polysaccharides are macromolecular complexes with multi-targeting bioactivities, low toxicity, and high safety. Some have been demonstrated to exhibit neuroprotective effects via their antioxidant, anti-amyloidogenic, anti-neuroinflammatory, anticholinesterase, anti-apoptotic, and anti-neurotoxicity activities, which have potential in the treatment of NDs. This review focuses on the different processes involved in ND development and progression, highlighting the neuroprotective activities and potential role of mushroom polysaccharides and summarizing the limitations and future perspectives of mushroom polysaccharides in the prevention and treatment of NDs.

Polyol and sugar osmolytes stabilize the molten globule state of α-lactalbumin and inhibit amyloid fibril formation

Protein misfolding and aggregation are associated with several human diseases such as Alzheimer's, Parkinson's, prion related disorders, type-II diabetes, etc. Different strategies using molecular chaperones, synthetic and naturally occurring small molecules, osmolytes, etc. have been used to prevent protein aggregation and amyloid fibril formation. In this study, we have used bovine α-lactalbumin at pH 1.6, 37 °C, and shaking conditions to promote amyloid fibril formation. Polyol and sugar osmolytes like glycerol, sorbitol, and trehalose have been used to inhibit the fibrillation of a number of proteins. In the present work, amyloid fibril formation of α-lactalbumin has been shown by ThT assay and AFM, while changes in the secondary structure during fibrillation has been followed by circular dichroism spectroscopy. Our results show that glycerol, sorbitol, and trehalose affect amyloid fibril formation of α-lactalbumin in a concentration-dependent manner. There is a delay in the lag phase of amyloid fibril formation in sorbitol and trehalose and complete inhibition in 6 M glycerol. Our results indicate that delay in the lag phase and inhibition of amyloid fibril formation are due to the stabilization of molten globule state by these osmolytes. At pH 1.6, the molten globule as well as the amyloid fibrils bind to ANS. However, when pH was shifted from 1.6 to 7, only the oligomeric and the fibrillar species bind to ANS due to refolding of molten globule state. The outcome of this study might be useful in designing small molecules which may stabilize the intermediate states, thus preventing amyloid fibril formation.

Natural Polysaccharides as Preventive and Therapeutic Horizon for Neurodegenerative Diseases

Neurodegenerative diseases are a serious and widespread global public health burden amongst aging populations. The total estimated worldwide global cost of dementia was US$818 billion in 2015 and has been projected to rise to 2 trillion US$ by 2030. While advances have been made to understand different neurodegenerative disease mechanisms, effective therapeutic strategies do not generally exist. Several drugs have been proposed in the last two decades for the treatment of different types of neurodegenerative diseases, with little therapeutic benefit, and often with severe adverse and side effects. Thus, the search for novel drugs with higher efficacy and fewer drawbacks is an ongoing challenge in the treatment of neurodegenerative disease. Several natural compounds including polysaccharides have demonstrated neuroprotective and even therapeutic effects. Natural polysaccharides are widely distributed in plants, animals, algae, bacterial and fungal species, and have received considerable attention for their wide-ranging bioactivity, including their antioxidant, anti-neuroinflammatory, anticholinesterase and anti-amyloidogenic effects. In this review, we summarize different mechanisms involved in neurodegenerative diseases and the neuroprotective effects of natural polysaccharides, highlighting their potential role in the prevention and therapy of neurodegenerative disease.

Chemical stabilization of γ-polyglutamate by chitosan and the effect of co-solvents on the stability

In protein-based formulations, conformational distortions and attractive interactions may cause insoluble and undesired aggregates. In the case of ionic peptides, including cationic or anionic, commonly electrostatic interactions are the main factors that control structure assembling. In this study, it was proposed that grafting of chitosan (CS) to γ-polyglutamic acid (γ-PGA) might exhibit much strong inhibiting effect on the formation of protein aggregates due to multiple amino groups and hydrophilic properties. To guarantee stable and safe biopharmaceutical formulation, the potency of a variety of stabilizers including sugars (glucose, sucrose), polyols (sorbitol, glycerol), surfactant (Tween 20), salting-out salt (PBS), and also different pH values have been evaluated on stabilizing or destabilizing the native state of CS-g-PGA copolymer using FTIR, CD, DLS, and SDS-PAGE. The comparable analysis revealed that the stability of CS-g-PGA was strongly dependent on pH owing to the polyelectrolyte characteristics of the polymers. Altogether these results implied that CS at optimized conditions might be an important precursor for the pharmaceutical industry and function as a new polymer for aggregation suppression and protein stabilization.

Destabilization potential of beta sheet breaker peptides on Abeta fibril structure: an insight from molecular dynamics simulation study

Alzheimer's disease is characterized by amyloid-β aggregation. Currently, all the approved medications are to treat the symptoms but there is no clinically approved treatment for the cure or to prevent the progression of Alzheimer's disease (AD). Earlier reports suggest the use of small molecules and peptides to target and destabilize the amyloid fibril. The use of Beta Sheet Breaker (BSB) peptides seems to be a promising and attractive therapeutic approach as it can strongly bind and destabilize the preformed amyloid fibril. There are experimental studies describing the destabilization role of various BSB peptides, but the exact mechanism remains elusive. In the current work, an attempt is made to study the destabilization mechanism of different BSB peptides on preformed amyloid protofibril using molecular docking and simulations. Molecular docking of eight different BSB peptides of varying length (5-mer to 10-mer) on the Abeta protofibril was done. Docking was followed by multiple sets of molecular simulations for the Abeta protofibril–BSB peptide complex for each of the top ranked poses of the eight BSB peptides. As a control, multiple sets of simulations for the Abeta protofibril (APO) were also carried out. An increase in the RMSD, decrease in the number of interchain hydrogen bonds, destabilization of important salt bridge interactions (D23–K28), and destabilization of interchain hydrophobic interactions suggested the destabilization of Abeta protofibril by BSB peptides. The MM-GBSA free energy of binding for each of the BSB peptides was calculated to measure the binding affinity of BSB peptides to Abeta protofibril. Further residue wise contribution of free energy of binding was also calculated. The study showed that 7-mer peptides tend to bind strongly to Abeta protofibril as compared to other BSB peptides. The KKLVFFA peptide showed better destabilization potential as compared to the other BSB peptides. The details about the destabilization mechanism of BSB peptides will help in the design of other peptides for the therapeutic intervention for AD.

Destabilzation of Abeta protofibril by Beta Sheet Breaker (BSB) peptides.

"What Doesn't Kill You Makes You Stronger": Future Applications of Amyloid Aggregates in Biomedicine

Amyloid proteins are linked to the pathogenesis of several diseases including Alzheimer's disease, but at the same time a range of functional amyloids are physiologically important in humans. Although the disease pathogenies have been associated with protein aggregation, the mechanisms and factors that lead to protein aggregation are not completely understood. Paradoxically, unique characteristics of amyloids provide new opportunities for engineering innovative materials with biomedical applications. In this review, we discuss not only outstanding advances in biomedical applications of amyloid peptides, but also the mechanism of amyloid aggregation, factors affecting the process, and core sequences driving the aggregation. We aim with this review to provide a useful manual for those who engineer amyloids for innovative medicine solutions.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.

Peptides as Potential Therapeutics for Alzheimer's Disease

Intracellular synthesis, folding, trafficking and degradation of proteins are controlled and integrated by proteostasis. The frequency of protein misfolding disorders in the human population, e.g., in Alzheimer's disease (AD), is increasing due to the aging population. AD treatment options are limited to symptomatic interventions that at best slow-down disease progression. The key biochemical change in AD is the excessive accumulation of per-se non-toxic and soluble amyloid peptides (Aβ(1-37/44), in the intracellular and extracellular space, that alters proteostasis and triggers Aβ modification (e.g., by reactive oxygen species (ROS)) into toxic intermediate, misfolded soluble Aβ peptides, Aβ dimers and Aβ oligomers. The toxic intermediate Aβ products aggregate into progressively less toxic and less soluble protofibrils, fibrils and senile plaques. This review focuses on peptides that inhibit toxic Aβ oligomerization, Aβ aggregation into fibrils, or stabilize Aβ peptides in non-toxic oligomers, and discusses their potential for AD treatment.

Unraveling the novel effects of aroma from small molecules in preventing hen egg white lysozyme amyloid fibril formation

This study investigated for the first time the molecular effectiveness of 'aroma' from three small molecules including a phenol (phenyl ethyl alcohol; PEA) and an aldehyde (cinnamaldehyde; Cin) both containing an aromatic ring, and a diamine (N,N,N,N'- Tetramethylethylenediamine; TEMED) at two different amounts (small; S and large; L) in preventing hen egg white lysozyme (HEWL) amyloid fibril formation using Thioflavin T and Nile red fluorescence assays, circular dichroism spectroscopy, SDS-polyacrylamide gel electrophoresis, atomic force microscopy, dynamic light scattering and HEWL activity test. Interestingly, the results revealed that (1) the aroma of PEA, identified as an active constituent of Rosa damascena, prevented fibril formation since PEA-L was able to trap the oligomeric form of HEWL in contrast to PEA-S where protofibrils but not mature fibrils were formed; (2) Cin, previously shown to prevent fibril formation in the liquid form, was also shown to do so in the aroma form by producing protofibrils and not mature fibrils in both Cin- L and Cin-S aroma forms and (3) the aroma of TEMED-L was able to retain HEWL's native structure completely and prevented both aggregation and fibril formation, while TEMED-S prevented HEWL fibril formation and instead directed the pathway towards amorphous aggregate formation. Furthermore, the ability to trap oligomeric species (by PEA-L aroma) is of great importance for further research as it provides routes for preventing the formation of toxic oligomeric intermediates along the fibrillation pathway. Last but not least, the novelty of this in vitro study on the effect of aroma at the molecular level with a unique experimental set-up using HEWL as a model protein in assessing amyloid fibril formation paves the way for more and detailed studies on the importance of aroma producing molecules and their effects.

Factors affecting the physical stability (aggregation) of peptide therapeutics

The number of biological therapeutic agents in the clinic and development pipeline has increased dramatically over the last decade and the number will undoubtedly continue to increase in the coming years. Despite this fact, there are considerable challenges in the development, production and formulation of such biologics particularly with respect to their physical stabilities. There are many cases where self-association to form either amorphous aggregates or highly structured fibrillar species limits their use. Here, we review the numerous factors that influence the physical stability of peptides including both intrinsic and external factors, wherever possible illustrating these with examples that are of therapeutic interest. The effects of sequence, concentration, pH, net charge, excipients, chemical degradation and modification, surfaces and interfaces, and impurities are all discussed. In addition, the effects of physical parameters such as pressure, temperature, agitation and lyophilization are described. We provide an overview of the structures of aggregates formed, as well as our current knowledge of the mechanisms for their formation.

Rationally Designed Peptides and Peptidomimetics as Inhibitors of Amyloid-β (Aβ) Aggregation: Potential Therapeutics of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease with no clinically accepted treatment to cure or halt its progression. The worldwide effort to develop peptide-based inhibitors of amyloid-β (Aβ) aggregation can be considered an unplanned combinatorial experiment. An understanding of what has been done and achieved may advance our understanding of AD pathology and the discovery of effective therapeutic agents. We review here the history of such peptide-based inhibitors, including those based on the Aβ sequence and those not derived from that sequence, containing both natural and unnatural amino acid building blocks. Peptide-based aggregation inhibitors hold significant promise for future AD therapy owing to their high selectivity, effectiveness, low toxicity, good tolerance, low accumulation in tissues, high chemical and biological diversity, possibility of rational design, and highly developed methods for analyzing their mode of action, proteolytic stability (modified peptides), and blood-brain barrier (BBB) permeability.

Modulation of Amyloid Aggregates into Nontoxic Coaggregates by Hydroxyquinoline Appended Polyfluorene

Inhibitory modulation toward de novo protein aggregation is likely to be a vital and promising therapeutic strategy for understanding the molecular etiology of amyloid related diseases such as Alzheimer's disease (AD). The building up of toxic oligomeric and fibrillar amyloid aggregates in the brain plays host to a downstream of events, causing damage to axons, dendrites, synapses, signaling, transmission, and finally cell death. Herein, we introduce a novel conjugated polymer (CP), hydroxyquinoline appended polyfluorene (PF-HQ), which has a typical "amyloid like" surface motif and exhibits inhibitory modulation effect on amyloid β (Aβ) aggregation. We delineate inhibitory effects of PF-HQ based on Thioflavin T (ThT) fluorescence, atomic force microscopy (AFM), circular dichroism (CD), and Fourier transform infrared (FTIR) studies. The amyloid-like PF-HQ forms nano coaggregates by templating with toxic amyloid intermediates and displays improved inhibitory impacts toward Aβ fibrillation and diminishes amyloid cytotoxicity. We have developed a CP based modulation strategy for the first time, which demonstrates beneficiary amyloid-like surface motif to interact efficiently with the protein, the pendant side groups to trap the toxic amyloid intermediates as well as optical signal to acquire the mechanistic insight.

Effects of hydroxylated carbon nanotubes on the aggregation of Aβ16-22 peptides: a combined simulation and experimental study

The pathogenesis of Alzheimer's disease (AD) is associated with the aggregation of amyloid-β (Aβ) peptides into toxic aggregates with ?-sheet character. In a previous computational study, we showed that pristine single-walled carbon nanotubes (SWCNTs) can inhibit the formation of β-sheet-rich oligomers in the central hydrophobic core fragment of Aβ (Aβ16-22). However, the poor solubility of SWCNTs in water hinders their use in biomedical applications and nanomedicine. Here, we investigate the influence of hydroxylated SWCNT, a water-soluble SWCNT derivative, on the aggregation of Aβ16-22 peptides using all-atom explicit-water replica exchange molecular dynamics simulations. Our results show that hydroxylated SWCNTs can significantly inhibit β-sheet formation and shift the conformations of Aβ16-22 oligomers from ordered β-sheet-rich structures toward disordered coil aggregates. Detailed analyses of the SWCNT-Aβ interaction reveal that the inhibition of β-sheet formation by hydroxylated SWCNTs mainly results from strong electrostatic interactions between the hydroxyl groups of SWCNTs and the positively charged residue K16 of Aβ16-22 and hydrophobic and aromatic stacking interactions between SWCNTs and F19 and F20. In addition, our atomic force microscopy and thioflavin T fluorescence experiments confirm the inhibitory effect of both pristine and hydroxylated SWCNTs on Aβ16-22 fibrillization, in support of our previous and present replica exchange molecular dynamics simulation results. These results demonstrate that hydroxylated SWCNTs efficiently inhibit the aggregation of Aβ16-22; in addition, they offer molecular insight into the inhibition mechanism, thus providing new clues for the design of therapeutic drugs against amyloidosis.

Antibody-conjugated, dual-modal, near-infrared fluorescent iron oxide nanoparticles for antiamyloidgenic activity and specific detection of amyloid-β fibrils

Amyloid-β (Aβ) peptide is the main fibrillar component of plaque deposits found in brains affected by Alzheimer’s disease (AD) and is related to the pathogenesis of AD. Passive anti-Aβ immunotherapy has emerged as a promising approach for the therapy of AD, based on the administration of specific anti-Aβ monoclonal antibodies (aAβmAbs) to delay Aβ aggregation in the brain. However, the main disadvantage of this approach is the required readministration of the aAβmAbs at frequent intervals. There are only a few reports describing in vitro study for the immobilization of aAβmAbs to nanoparticles as potential targeting agents of Aβ aggregates. In this article, we report the immobilization of the aAβmAb clone BAM10 to near-infrared fluorescent maghemite nanoparticles for the inhibition of Aβ₄₀ fibrillation kinetics and the specific detection of Aβ₄₀ fibrils. The BAM10-conjugated iron oxide nanoparticles were well-characterized, including their immunogold labeling and cytotoxic effect on PC-12 (pheochromocytoma cell line). Indeed, these antibody-conjugated nanoparticles significantly inhibit the Aβ₄₀ fibrillation kinetics compared with the same concentration, or even five times higher, of the free BAM10. This inhibitory effect was confirmed by different assays such as the photo-induced crosslinking of unmodified proteins combined with sodium dodecyl sulfate– polyacrylamide gel electrophoresis. A cell viability assay also confirmed that these antibody-conjugated nanoparticles significantly reduced the Aβ₄₀-induced cytotoxicity to PC-12 cells. Furthermore, the selective labeling of the Aβ₄₀ fibrils with the BAM10-conjugated near-infrared fluorescent iron oxide nanoparticles enabled specific detection of Aβ₄₀ fibrils ex vivo by both magnetic resonance imaging and fluorescence imaging. This study highlights the immobilization of the aAβmAb to dual-modal nanoparticles as a potential approach for aAβmAb delivery, eliminating the issue of readministration, and contributes to the development of multifunctional agents for diagnosis and therapy of AD.

Computational design of new Peptide inhibitors for amyloid Beta (aβ) aggregation in Alzheimer's disease: application of a novel methodology

Alzheimer's disease is the most common form of dementia. It is a neurodegenerative and incurable disease that is associated with the tight packing of amyloid fibrils. This packing is facilitated by the compatibility of the ridges and grooves on the amyloid surface. The GxMxG motif is the major factor creating the compatibility between two amyloid surfaces, making it an important target for the design of amyloid aggregation inhibitors. In this study, a peptide, experimentally proven to bind Aβ40 fibrils at the GxMxG motif, was mutated by a novel methodology that systematically replaces amino acids with residues that share similar chemical characteristics and subsequently assesses the energetic favorability of these mutations by docking. Successive mutations are combined and reassessed via docking to a desired level of refinement. This methodology is both fast and efficient in providing potential inhibitors. Its efficiency lies in the fact that it does not perform all possible combinations of mutations, therefore decreasing the computational time drastically. The binding free energies of the experimentally studied reference peptide and its three top scoring derivatives were evaluated as a final assessment/valuation. The potential of mean forces (PMFs) were calculated by applying the Jarzynski‚s equality to results of steered molecular dynamics simulations. For all of the top scoring derivatives, the PMFs showed higher binding free energies than the reference peptide substantiating the usage of the introduced methodology to drug design.